Radio frequency identification systems are radio communication systems used for identifying and tracking objects for a variety of applications, such as automatic toll collection, electronic shelf labelling, management of the stocks in a store, etc. In such systems, communication takes place between an interrogating station (a radio transceiver) and tags (transponders) applied to the individual objects to be identified and tracked and carrying suitable information about the object (e.g. serial number, price . . . ).
The tags may be active or passive. An active tag synthesises a radio frequency carrier that is modulated by the information recorded on the tag itself and is transmitted to the interrogating station. In case of passive tags, the interrogating station transmits a radio frequency inquiry signal to the tag, which uses the inquiry signal to energise its antenna and reflects the received signal to the interrogating station, after having modulated such signal by the information recorded on the tag. Passive tags are used whenever a low cost of the tag is an essential condition, as is for instance the case of RFID systems intended to manage stocks in a store. The present invention concerns RFID systems with passive tags.
A problem encountered with many conventional RFID systems using passive tags is the limited range operation, which typically is less than 1 m, e.g. about 80 cm. This is due to the need of using omnidirectional antennas in order the interrogating station can interrogate tags scattered in the space where the system operates. Thus the tags receive (and therefore reflect) only a small fraction of the total power transmitted by the interrogating station. Moreover, the signals transmitted by the interrogating station and by the tags may undergo reflections by the walls, the shelves on which the objects are located, etc., so that they are affected by a lot of noise. Thus, if the distance between the interrogating station and the tags is too long, the signals could be too weak to activate the tags and the signal-to-noise ratio could be too low to allow reliable detection at the interrogating station of the information transmitted by a tag.
Some solutions have already been proposed to extend the operating range of passive tag RFID systems.
For instance, U.S. Pat. No. 6,184,841 B1 discloses a system, especially intended for tracking cargo containers, in which the interrogating station uses a single transmitting planar antenna and a receiving antenna consisting in a linear array of planar antennas. The arrangement is such that the horizontal width of the uplink beam is greatly reduced with respect to the horizontal width of the downlink beam. For instance, a horizontal beamwidth of 30° is indicated for the uplink beam, whereas the downlink beam has a horizontal beamwidth of 60°. The reduced beamwidth results in a higher antenna gain, improving the performance of the uplink signal as to both reliability and range of operation, which is extended up to some metres.
The known system however is still unsatisfactory. It requires two different antenna systems at the interrogating station, and this makes the system more complex and therefore more expensive. Moreover, the system can only detect tags on the line-of-sight of the antenna: the system can be used for identifying objects when passing by the interrogating station, but it is not able to track them while being displaced within a warehouse, a container or the like.
The document “An active integrated retrodirective transponder for remote information retrieval-on-demand”, by R. Y. Miyamoto et al., IEEE Transactions on Microwave Theory and Technique”, 49(9), pages 1658-1662, September 2001, discloses a RF tag including active circuitry that effects phase conjugation of inquiry signals sent by a base station, as well as an amplification of the signals sent back to the base station. This solution allows increasing the operating range and dispensing with the need for the tag to be in the line-of-sight of the antenna of the base station. Yet a system using phase conjugators in the tags is complex and expensive. Moreover, there is the need of powering each tag.
Thus, the invention aims at solving the above problems, by providing a method of and an apparatus for reading tags in a RFID system by using an antenna array, which system allows a long range of operation without need for active circuitry in tags, and does not require that the tag is on the line-of-sight of the antenna of the interrogating station, so that a tracking of the tagged objects is possible.